The combination of polymer chains and gels, commonly referred to as “snake-in-a cage” or “snake-cage” systems, has been reported previously, and such combinations have been used to achieve various purposes.
In 1957, Hatch et al. disclosed a new type of ion-exchange resin, which they termed a snake-cage polyelectrolyte (Hatch et al., Industrial and Engineering Chemistry (1957), vol. 49, 1812-19). This compound consisted of a crosslinked polymer network carrying a fixed charge, in which was contained a physically trapped linear polyelectrolyte of the opposite charge. The crosslinked network used in this work was a commercial cationic resin, Dowex 1, constituting the “cage”, in which the linear anionic polymer “snake” was trapped. Amphoteric polymeric compositions prepared from snake-cage resins were later disclosed in U.S. Pat. No. 3,875,085.
The snake-cage polyelectrolytes have some unusual properties through which they differ from conventional ion-exchangers. One of these properties is that they reversibly absorb both anions and cations simultaneously from aqueous solutions of electrolytes. The absorbed electrolytes can then be removed by merely washing with water. Unlike conventional ion-exchange resins, the snake-cage resins swell in concentrated solutions and shrink in diluted solutions.
While attempts were made to entrap polyelectrolytes in neutral cages, these attempts all met with failure unless the polyelectrolyte was grafted into the cage network or unless the polyelectrolyte was crosslinked. In the absence of grafting or crosslinking, the polyelectrolytes could be extracted almost completely from a neutral cage-forming polymer. In one example of such a system, an ionically crosslinked polyelectrolyte was entrapped in a poly(vinyl alcohol) gel to enhance the water vapor transport through a membrane (WO 02/38257). Again, in the absence of crosslinking, the polyelectrolyte could be extracted from the neutral poly(vinyl alcohol) gel on standing in water.
It is of note that the previously reported snake-in-cage systems consisted of dense polymeric gels, rather than macroporous gels, as will be presently discussed.
Improvement of mechanical properties of hydrogels by encasing a hydrophobic polymer in the gel network was also reported by Nagaoka (Polymer Journal (1989), vol 21, 847-850). Hydrogels consisting of hydrophilic crosslinked poly(N-vinyl-2-pyrrolidone) encasing hydrophobic polymers of either cellulose acetate, poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl butyral), polyurethane, or polystyrene, were synthesized and evaluated for use in medical applications (optical lenses), and were compared with poly(2-hydroxyethyl methacrylate), currently used in these applications. The hydrogels were synthesized using the snake-in-cage method, in which a specified amount of the hydrophobic polymer was dissolved in N-vinyl-2-pyrrolidone (monomer) containing diethylene glycol dimethacrylate (crosslinker), and polymerized. Based on measurements of tensile strength, elongation at break, initial tensile modulus, and H2O content, hydrogels incorporating either cellulose acetate or poly(methyl methacrylate) were superior to poly(2-hydroxyethyl methacrylate). The transmittance of visible light of 500 nm through the hydrogels of thickness 100 μm was greater than 98%, and hydrogels also displayed H2O permeability of over ten times that of poly(2-hydroxyethyl methacrylate). The high light transparency of these snake-in-cage hydrogels indicates that the gels were homogeneous as they need to be to work as optical lenses. The water permeability given in the paper also confirms this conclusion, as the reported water permeability, converted into Darcy hydrodynamic permeability, was in the order of 1.2×10−20 m2. Such low permeability is comparable with that of very dense gel-filled membranes that were previously reported by Mika et al. (Industrial and Engineering Chemistry Research (2001), vol. 40, 1694-1705), and as such these gels represent a different class of materials than those presently disclosed.